Replace caller-depth guard with O(1) re-entrancy detection

lib/errgonomic.rb

@@ -46,6 +46,11 @@ class ArgumentError < Error; end
 
   class ResultRequiredError < Error; end
 
+  # Raised when a wrapped ActiveRecord attribute reader re-enters itself on
+  # the same record, catching runaway recursion at the first repeated frame
+  # instead of a SystemStackError thousands of frames later.
+  class RecursiveOptionalReadError < Error; end
+
   class NotComparableError < StandardError; end
 
   class SerializeError < TypeError; end

lib/errgonomic/rails.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-require_relative 'option'
+require_relative '../errgonomic'
 require_relative 'rails/active_record_optional'
 require_relative 'rails/active_record_delegate_optional'
 

lib/errgonomic/rails/active_record_optional.rb

@@ -18,8 +18,19 @@ module ActiveRecordOptional
         @errgonomic_optionals.each do |name|
           class_eval <<-RUBY, __FILE__, __LINE__ + 1
             def #{name}
-              raise "stack too deep" if caller.length > 1024
-              val = super
+              reads = Thread.current[:errgonomic_optional_reads] ||= {}
+              key = [object_id, :#{name}]
+              if reads[key]
+                raise Errgonomic::RecursiveOptionalReadError,
+                      "\#{self.class}##{name} re-entered itself; something beneath this reader reads it again"
+              end
+
+              reads[key] = true
+              begin
+                val = super
+              ensure
+                reads.delete(key)
+              end
               val.nil? ? Errgonomic::Option::None.new : Errgonomic::Option::Some.new(val)
             end
           RUBY

test/rails_test.rb

@@ -57,6 +57,21 @@ class Genre < ActiveRecord::Base
   delegate_optional :name, to: :parent, prefix: true, private: true
 end
 
+# A buggy layer that re-enters the attribute reader from beneath it: the
+# generated reader's super lands here, and the unqualified call restarts
+# dispatch at the top of the chain.
+module ReentrantBio
+  def bio
+    bio
+  end
+end
+
+class LoopyAuthor < ActiveRecord::Base
+  self.table_name = 'authors'
+  include ReentrantBio
+  include Errgonomic::Rails::ActiveRecordOptional
+end
+
 class BugTest < Minitest::Test
   def test_optional_attributes
     author = Author.create!(name: 'Cixin Liu')
@@ -88,10 +103,31 @@ def test_delegate_optional
     assert_equal author.name, book.author_name.unwrap!
   end
 
+  # Wrapped readers must work on a deep-but-legitimate stack; genuine runaway
+  # recursion is SystemStackError's job.
+  def test_optional_attributes_read_on_a_deep_stack
+    author = Author.create!(name: 'Cixin Liu', bio: 'writes sci-fi')
+    deeper(1100) { assert_equal 'writes sci-fi', author.bio.unwrap! }
+  end
+
+  def test_recursive_read_raises_a_named_error_at_first_reentry
+    author = LoopyAuthor.create!(name: 'Cixin Liu', bio: 'writes sci-fi')
+    error = assert_raises(Errgonomic::RecursiveOptionalReadError) { author.bio }
+    assert_match(/bio/, error.message)
+  end
+
   def test_private_delegate_optional
     fiction = Genre.create!(name: 'Fiction')
     scifi = Genre.create!(name: 'Sci-Fi', parent: fiction)
     assert_raises(NoMethodError) { scifi.parent_name }
     assert_equal 'Fiction', scifi.send(:parent_name).unwrap!
   end
+
+  private
+
+  def deeper(frames, &block)
+    return block.call if frames.zero?
+
+    deeper(frames - 1, &block)
+  end
 end